In connection with the conventional method of keeping a geostationary satellite in position, the satellite should continuously be located in a predetermined longitudinal and latitudinal area within the so-called tolerance window. To assure this, course surveys, called orbiting, are performed in the prior art at suitable times by means of one or several appropriately equipped ground stations, with the aid of which the exact satellite position is determined and predicted, i.e. orbit determinations and predictions are performed. In order to prevent a departure from the tolerance window, caused by natural or forced orbit perturbations, i.e. to assure station keeping, orbit correction maneuvers are calculated in the ground control center and are transmitted for execution to the satellite by a ground station. The required command sequence is generated on the ground, checked and transmitted to the satellite. The reaction of the satellite is checked in each intermediate step from its telemetry data.
Because of increased utilization and subdivision of the geostationary orbit, however, common tolerance windows are assigned to several satellites by the responsible authorities, i.e. the satellites are co-positioned. Tests have shown that because of an uncoordinated station keeping within a tolerance window a risk of collisions exists for the satellites, which cannot be neglected.
The risk of collisions can be considerably reduced by appropriately coordinated station keeping strategies, which provide defined differences, setting the so-called nominal relative trajectories, between the orbital elements of individual satellites. Frequently discussed strategies utilize a longitudinal separation, an eccentricity vector separation or a coordinated eccentricity/inclination vector separation. For example, the French telecommunications satellites TDF-1 and TDF-2 at 18.8 W. longitude are maintained in a tolerance window of a size of 0.2.times.0.2 degrees by means of different eccentricity vectors. The British satellites BSB-1 and BSB-2 at 31 W. longitude are the subject of a coordinated eccentricity/inclination vector separation.
Orbiting by ground stations generally result in relatively large orbit determination errors regarding the longitudinal position of the satellites. This disadvantageous effect is increased by orbit correction maneuvers, in particular inclination maneuvers, which in the course of their execution are also error-prone in regard to size and direction, in that an additional unintentional longitudinal drift can occur. Safety, which is reduced for these reasons, requires larger amounts in the separation between the orbit elements eccentricity and inclination in connection with station keeping of several satellites within a tolerance window which, however, results in an unnecessarily extensive utilization of the available area of the tolerance window and limits the possible number of satellites per window.
A further disadvantage of orbiting by ground stations lies in that the geometry of the position of a geostationary satellite to be surveyed only changes little and slowly, namely with a period of one day. Therefore a time expenditure of approximately two days is required today for a sufficiently accurate orbit determination. But more accurate and rapid orbit survey or orbit determination methods are necessary in order to do justice over time to the increasing number of satellites in the geostationary orbit.
Practical experience has shown that the coordinated station keeping within a tolerance window, whose area was assigned to different countries for use, is made more difficult for a number of reasons. For example, in most cases satellites operated by different countries are surveyed and controlled by different ground stations. This decentralized control results in that the station keeping methods, which are the same in principle, are multiplied, and additional efforts are required for the coordination of the ground control centers, for example for matching the tracking systems and calculation programs. In turn, this results in a large expenditure of time, elaborate data transfers and considerable costs. At the same time the range of possible error sources is increased. Furthermore, the required detailed command sequence on the ground for executing an orbit maneuver, and checking it step-by-step from the ground are extremely expensive.
In addition to this general prior art, a method for the coordinated station keeping of a satellite cluster in one position is proposed in German Published, Non-Examined Patent Application DE-OS 42 43 395 A1, wherein only a single satellite in the cluster is surveyed and controlled from the ground. The other satellites are subject to the control of this satellite, identified as MASCOT (Multiple Application Satellite for Cluster Control and Operational Tasks) which, besides its actual task as, for example, a telecommunications satellite, in addition takes over control of the entire cluster. In this case all satellites of the cluster, except for MASCOT, are separated from each other by means of combined eccentricity and inclination vector separation, this means, each satellite moves on an elliptical orbit around the earth, wherein the degrees of longitude of the points closest and farthest from earth of the orbits of these satellites, without taking the intrinsic rotation of the earth into account, are different. For an observer rotating along with the earth, at different times of day the satellites appear to be sequentially closest to or farthest away from the earth. Since furthermore a satellite moving on an elliptical orbit always covers equal areas in the equal intervals of time with its local vector originating from the center of rotation, periodically repeated deviations from the mean position of the satellite regarding its geographical longitude can also be detected by an observer turning with the earth. It can be observed inside the satellite cluster that, in a superimposition on an ideal circular orbit, the individual satellites move around the earth on an ellipse, on which the satellites of the cluster circle each other in the course of a day. In order to avoid observation, the orbital plane of each satellite is inclined differently in respect to the equatorial plane of the earth, because of which the satellites circle each other on an ellipse which is inclined in respect to the equator, because of which each satellite always has an unimpeded dissemination path in the direction toward the earth. As an essential part of this strategy, the MASCOT is positioned on this ellipse in such a way, that it can determine its relative position in respect to each other satellite of the cluster by means of optical or microwave radar, and furthermore can exchange data with each satellite of the cluster by means of wireless.
Thus, MASCOT has been assigned the task of controlling all remaining satellites and, if required, to initiate corrections in the trajectory of individual satellites in respect to MASCOT. Furthermore, collective corrections regarding the entire satellite cluster are executed via MASCOT, whose position is surveyed by a ground station and represents the position of the satellite cluster. Commands for orbit corrections are routed via this satellite to all the others. wherein the position of the other satellites in the cluster are surveyed and, if necessary, corrected through MASCOT. Since most of the installations required for these operations are installed in MASCOT, at least one reserve system is provided for each satellite cluster in order not to lose the entire satellite cluster in case of a malfunction.
An essential disadvantage of this method lies in the central importance of a single satellite, provided with special equipment, wherein possible savings in connection with the other satellites of the cluster must be weighed against the mission reliability. Furthermore, the arrangement of the satellites in respect to each other is basically limited to the combined application of the eccentricity and inclination vector separation, and an per se ideal arrangement on tightly staggered geographical longitudes is not possible because of the lack of transmission paths past the respectively adjoining satellites.
Thus, a specially embodied satellite is always required for taking on the master function, i.e. at least one further master must be placed in the cluster in order to take over the replacement function if required, for example when the actual master fails.